1. Field of the Invention
The present invention relates generally to a lithographic apparatus and a device manufacturing method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In a lithographic process, dust on or defects in the mask that are imaged onto the substrate can cause defects in the manufactured device and so masks are manufactured and stored with great care to avoid damage. Pellicles are conventionally used to prevent dust settling on the mask. A pellicle is a thin transparent sheet mounted on a frame so that it is spaced a few mm from the mask. Any dust particles settle on the pellicle rather than the mask and will be out of focus when the mask is imaged on the substrate so that they are not printed in the resist. For EUV, no material suitable for forming a pellicle is known and masks are based on multi-layer reflectors which are hard to repair so another approach to protecting the mask from dust and damage is required.